Once the thermoplastic film is held in place against the drum, and as the drum rotates, an electrically-heated hot wire or hot knife comes out of the drum, cuts through the film, and moves into the gap between the seal bars in order to seal and form the sides of the bag. In order to ensure that the film is cut and sealed, the gap between the seal bars must be large enough to enable the hot wire or hot knife to pass through the film.
The seal bars cannot be fixed in place. Rather, the seal bars must have a little "give", because the machine must be able to compensate for inconsistencies in the seal drum surface and movement of the seal drum while at the same time ensuring that the film is clamped between the clamping assembly and the seal drum.
A slight "give" is provided for in the prior art. U.S. Pat. No. 4,820,254 describes a clamping mechanism which has two seal bar segments defining a gap therebetween, whereby the seal bar segments could pivot toward one another. However, when the seal bar segments pivot toward each other, the gap between seal bar segments can become too small. Since the wire is electrically heated, if the gap is so narrow that the wire contacts a surface of a seal bar segment, the wire can short out and shut down the entire bag-making operation.
U.S. Pat. No. 5,211,619 describes a modest improvement. That patent describes a channel with a small clearance built in to permit the seal bar segments to pivot not only with respect to each other, but also with respect to the seal drum. However, the amount of pivoting is very low--0.025" to 0.050". With such a small amount of pivoting to compensate for drum movement, the clearances are very small and tedious precision is required to align all of the pieces. Thus, installation of such a seal bar is time consuming and tedious.
It would be an advance in the art of bag sealing devices if a clamping mechanism were invented which would preserve the gap between the seal bar segments while at the same time allowing adequate "give" between the clamping device and the seal drum. At the same time, it would be an advance in the art of bag sealing devices if a clamping apparatus were invented which would be self-aligning, so that the clamping assembly is efficient to install.